Device for the displacement of a solidification solid-liquid interface

ABSTRACT

Device for growing monocrystals of a material comprising an elongated container for the material and two ovens through which the container is circulated. A first oven maintains the position of the material which confronts it at a temperature close to the phase change point. The second oven, located before the first, heats the material to the liquid state. The temperature gradient in the interface zone is stabilized by displacing the second oven with respect to the first.

United States Patent [72] Inventors Jenn Glllet St-Mnrtin D'lieres; Yves Malmejnc, Grenoble, both of, France [2| Appl. No. 879,761 [22] Filed Nov. 25, 1969 [45] Patented July 13, 1971 [73} Assignee Commissarlat I L'Energle Atomique Paris, France [32] Priority Dec. 18, 1968 [3 3] m [3| 178,906

[54] DEVICE FOR THE DISPLACEMENT OF A SOLIDIFICA'IION SOLID-LIQUID INTERFACE 6 Claims, 5 Drawing Figs.

(52] US. Cl. 266/24, 23/273 SP, 23/30! SP, l48/L6 [St] MCI. ..C22b 41/00 [50] Field of Search 266/24, 34 R, 39; 148/] .6; 75/65 ZM; 23/273 SP, 301 SP; l3/DIG. l

[56] References Cited UNITED STATES PATENTS 2,876,147 3/l959 Kniepkamp et al l48/l .5 3,410,665 1l/l968 Muller et a]. 23/273 Primary Examiner-Gerald A. Dost AttorneyCameron, Kerkam & Sutton ABSTRACT: Device for growing monocrystals of a material comprising an elongated container for the material and two ovens through which the container is circulated. A first oven maintains the position of the material which confronts it at a temperature close to the phase change point. The second oven, located before the first, heats the material to the liquid state. The temperature gradient in the interface zone is stabilized by displacing the second oven with respect to the first.

PATENIEnJuu 31am 3.592.455

SHEU 1 0F 3 FIG] PATENIEB JUL 1 3 I911 SHEET 2 BF 3 vol & 5 QIJWI moi WW NM .0 NW u M m 5 3 m .1 I m 1 gm Q \u R wk (2a DEVICE FOR THE DISPLACEMENT OF A SOLIDIFICATION SOLID-LIQUID INTERFACE This invention is concerned with a device for displacing a solid-liquid interface along a mass of material in order to produce unidirectional growth of the solid phase. This device can be employed especially for the preparation by crystallization of semiconductor materials, of crystals having only a low dislocation density as well as materials of ultrahigh purity and large size, especially by crystallization in a metallic solvent.

There are already a large number of methods for producing crystal growth from a mass of molten material. it may be useful to recall that the phenomenon of unidirectional growth is dependent on a large number of interrelated parameters. However, these parameters can be considered as belonging to two different groups: the first group essentially involves parameters which are related to thermal phenomena and the second group involves parameters which express phenomena other than those of a thermal nature. Some of the first parame' ters (such as the temperature gradient 6, within the solid near the solidliquid interface, the temperature gradient 6 within the liquid near the same interface and the solidification rate V) are determined as a result of experimental conditions imposed from the exterior whereas other parameters (such as the coefficient of thermal conductivity K of the solid phase, the thermal conductivity K, of the liquid phase, the latent heat of solidification L of the material and the temperature T of change of phase) relate to the constituents. The parameters of the second group are especially the coefficients D, and D. of diffusion of the solute or solutes in the solid and liquid phases, the concentration C, of the solute or solutes at the level of the interface and the slope in of the liquidus of the phase diagram in the vicinity of the concentration C of the solutes in the liquid.

It is also known that the quality of the results obtained during progressive solidification is largely determined by the presence or absence of supersaturation (undercooling): when there is no supersaturation, the interface is substantially flat and smooth. When supersaturation is present and particularly at high values of this latter, the surface becomes pitted, fibrous, cellular or even dendritic. Utilizing the notations provided above, the following relation has been given as an essential condition for ensuring absence of supersaturation or underoooling:

II ZfW, o 1. However, it is possible in the course of the operation to modify only those parameters which have been fixed by the external conditions in dependence on the operator, that is to say in particular G, and V which do not directly depend on the reaction medium the formula given above clearly shows the advantage of extremely accurate control or these parameters.

It may be noted from a study of the prior art that the different processes and devices which have been employed up to the present time do not readily lend themselves to accurate and stable control of the parameters 6, and V. The wellknown Bridgman method of growing crystals can be mentioncd by way of example. In the oldest form of this method, a vessel containing a mass of liquid material is moved vertically downwards through a temperature gradient which is imposes from the exterior and the solidification front of the liquid is displaced upwards within the mass. The thermal gradient is established either by means of a single heating furnace (in which case the temperature gradient is present between said furnace and the surrounding atmosphere) or by means of two hot zones at different temperatures or within the interior of a furnace having a thermal profile which is determined by means of a variable number of heating elements. In a device of this type, the solidification rate is determined by the rate of flow of heat (this rate being in turn a function of the rate of transfer through the temperature gradient). Assuming by way of example that the vessel is withdrawn at a speed which is higher than the rate of flow of heat within the liquid portion of the mass, it is accordingly appircnt that the iiilid liquisl interface is displaced towards that region in which the cfl ilfi l' 'lt' of the mass is colder: in other words, the rate of solidificatiin this case is lower than the rate of mechanical withdrawal. The same general data remain valid in the alternative technique which is known as the Chalmers method in which the operation is carried out by horizontal displacement.

It is apparent that these different methods and variants of such methods all carry a series of disadvantages and notably the following:

In view of the fact that the stability ofthe gradient depends on the stability of temperatures of at least two heat sources (whether these are two hot zones or a hot zone and the surrounding atmosphere), two temperature regulations will be necessary in order to establish the gradient and their interaction is liable to result in hunting effects.

Stabilization of the gradient can be obtained only in a steady state of heat flow by reason of thermal inertia.

The methods referred to above are relatively costly and are difficult to apply in practice when it is necessary to produce substantial temperature gradients since the whole quantity of liquid must then be heated to the maximum temperature.

It is possible only to stabilize the gradient in the liquid phase but not to stabilize the gradient in the solid phase near the interface.

Except in the case of extremely low rates of withdrawal, the temperature gradient within the liquid region is not constant since the temperature distribution is not linear.

During a same operation involving growth of the solid phase by withdrawal of the vessel, the thermal flux equilibrium equations are disturbed by the end effects since the dimensions of the vessel are evidently finite and by the fact that the heat capacity of the vessel and material considered as a whole is always ofhigh value.

Practically speaking, the rate of withdrawal in devices of the prior art does not correspond to the solidification rate and ratios of the order of l:3 between these two rates are attained in some cases. In short, it is both extremely difficult and unsatisfactory to ensure accurate and continuous adjustment of the parameters which can be modified in order to eliminate or limit supersaturation.

The main object of the invention is to provide a device which removes or attenuates to a substantial extent the disadvantages mentioned above or at least the most serious disadvantages. Accordingly, the invention proposes a device for displacing a solidliquid interface along a mass of material, wherein said device comprises in combination: a container which defines a housing of elongated shape for receiving said mass of material; means for displacing said container along the axis of the housing; a first heating furnace having low thermal inertia interposed on the path of said container and equipped with regulating means for maintaining the temperature of the mass at the level of said furnace at a value close to that of the point of change of phase; a second furnace having higher power and thermal inertia than those of the first furnace, said second furnace being interposed on said path upstream of the furnace which has low thermal inertia and being intended to maintain the mass within the container at the level of said furnace at a temperature above that of the point of change of phase; means for displacing said second furnace relative to the first and regulating means for controlling the displacement of said furnace and maintaining the temperature gradient at a constant value within the liquid phase in proximity to the solidification interface.

in a preferred but nonlimitative construction, the second furnace ensures within the mass of material a temperature distribution having a maximum value which is displaced towards the first furnace with respect to the midplane of the second furnace. It is possible by this means to produce a substantial temperature gradient within the liquid phase in proximity to the solidification interface without thereby entailing the need to heat the entire liquid phase to the maximum temperature.

The arrangements which have just been defined ensure high stability in the growth rate and accurate control of the tem' perature gradient at least in the liquid phase and in proximity to the interface while only making use of very simple means.

A better understanding of the invention will be gained from the following description of one embodiment which is given by way of example but does not imply any limitation, reference being made to the accompanying drawings in which:

FIG. I is a very diagrammatic view in perspective showing the mechanical portions of the device;

FIG. 2 is a view in elevation showing the container of the device of FIG. 1;

FIG. 3 is an exploded view showing in perspective the casing of the container, the cradle, the screens and the cover (this latter being shown upside down);

FIG. 4 is a highly simplified block diagram of the regulating means which are associated with the device of FIG. 1;

FIG. 5 shows very diagrammatically a possible temperature distribution along the axis of displacement of the container of the device of FIG I, the outline ofthe container and furnaces being shown in dashed lines for the sake ofgreater clarity.

The device which is illustrated in FIG I comprises a stir tionary frame formed of a number of assembled parts and generally designated by the reference numeral It), said frame comprises a horizontal table on which are mounted flanges fut supporting the moving parts.

The material to be treated is placed within a container and intended to be maintained at a preselected temperature by means of de vices which essentially comprise a holding furnace I], a main furnace I4 and a cold furnace I6 which will be described in turn.

The holding furnace I2 is of annular shape so as to provide a passageway for the container, is of small thickness and is fixed on the table in the form of construction which is illus' trated in FIG. Since its function is limited to the supply of auxiliary heating power for regulating purposes, this furnace has only low thermal inertia. The furnace 12 is fitted with a regulating device to which further reference will be made hereinafter and which is intended to permit of rapid and accurate regulation of the temperature applied to the material by the holding furnace at the level of this latter.

The main furnace 14 which is also oi annular shape is mounted coaxially with the holding furnace l2 and has a ther mal power rating which is distinctly higher As will be ex plained hereinafter, this furnace is fitted with a device [5 (shown in FIG. 4) which provides relatively coarse control for the purpose of temperature stabilization. The main furnace I4 is capable of displacement with respect to the holding furnace 12 under the action ofa mechanism controlled by a regulating device which will be described later and is intended to maintain within the liquid phase in proximity to the liquid-solid interface a constant and predetermined temperature gradient within the material to be treated The mechanism for displacing the main furnace I4 comprises two threaded rods I7 actuated by a single servomotor 18 by means of a gear train 20. The ends of the two threaded rods 17 are adapted to rotate in bearings provided for this purpose in two parallel and vertical support brackets 22 and 22' which are mounted on the stationary table. The frame of the furnace I4 is provided with lateral junction gussets 24 and these latter are adapted to carry internally threaded sleeves 26 into which the threaded rods [7 are screwed. The rotation of the threaded rods is thus intended to displace the furnace I4 towards the holding furnace 12 or to move it away from this latter, depending on the direction of rotation of the rods and therefore of the servomo tor 18.

The holding furnace l2 and the furnace [4 can be of different types, the choice being dependent in particular on the nature of the material to be treated. It is possible in particular to employ resistance furnaces. In other cases, it may be preferably to employ high-frequency induction furnaces since this expedient permits direct evolution of heat from the material to be treated provided that this latter is electrically conductive No matter what type of main furnace may be adopted, it is preferable to ensure that said furnace has a temperature dislrihutioii along its axis which is not symmetrical but has a maximum value which is displaced towards the holding furnace with respect to its midplane as indicated in FIG. 5. By producing the maximum temperature which is necessary within the material at a shorter distance from the interface, this arrange merit in fact makes it possible to reduce the total thermal power to be dissipated within the whole quantity of material.

In the form ofconstruction which is illustrated in FIGv l, the cold furnace 16 consists simply of a sleeve which is coaxial with the furnaces I2 and I4 and which is capable of axial dis placement. Said sleeve is carried by a tubular traction rod 30 which is guided by the support bracket 22' and slidably fitted within this latter. Said traction rod is provided with a connector 32 for the admission of a cooling fluid which is supplied through a nozzle 34 and for the discharge of said coolant through a nozzle 36.

The mechanism which serves to actuate the cold furnace I6 is similar in construction to the mechanism which is associated with the main furnace 14. This mechanism comprises a yoke 38 which is secured to the traction rod 30 and provided with internally threaded bores. Two threaded rods 40 which are parallel to the traction rod 30 are screwed into said bores. The rods 40 are secured against translational motion within two support brackets 42 and 42' and are driven in rotation by an assembly consisting of a seriomotor, gear box and gear train as shown diagrammatically at 44 In the form ofconstruction which is illustrated in FIG. I, the cold furnace to is secured to the terminal portion ofthe container 46 (which is illustrated in FIGS. 2 and 3). The material to be treated is placed within said container so that the mechanism for actuating the furnace 16 also constitutes the mechanism for withdrawing the container 46, that is to say for displacing along the container the solidification front of the material which is present in said container.

A certain number of conditions must be satisfied by the con tainer in order to ensure that no disturbance is introduced in the flow ofheat. In particular:

the container must have perfect thermal symmetry in order to minimize the harmful effects of convection through the container wall;

the container must provide the material which is present thciciii with at least partial protection against disturbing exter rial lflflllCl'iCtn in particular in the intermediate zone between the furnaces. this entails the need to ensure satisfactory heat insulation of the material with respect to the surrounding atmosphere, while nevertheless permitting (in the case of fur iiaces other than induction furnaces) the transmission of heat from these furnaces to the material. This result is achieved by adopting for the fabrication of the container a material which has low thermal conductivity with respect to the conductivity of the material to be treated;

finally, the container must permit of accurate temperature measurement within the material to be treated while ensuring that these measurements are not liable to disturb the transmissioii and homogeneity ofthe thermal flux.

The container which is illustrated in FIGS. 2 and 3 satisfies these conditions. After assembly (as shown in FIG. 2), said container assumes the form of a main portion having a uniform hexagonal cross section and having sufficient symmetry about its axis to prevent any inhomogeneity about said axis, said main portion being provided with an extension in the form of a cylindrical end fitting 48. Said end fitting is adapted to engage in the sleeve which constitutes the cold furnace I6 and is secured therein by means of a locking pin which is inserted in the radial bore 50 (shown in FIG. I) and the radial bore 52 (shown in FIG. 2).

The main portion of the container comprises a casing 54 which is rigidly fixed to the cylindrical end fitting 48 and forms a flat bottomed trough 56. A cradle 58 is placed in said trough. After loading with material to be treated, the cradle 58 is covered with two thermal screens 60 and 60' and then placed within the trough 66. The casing is then closed by a cover 62 (which is shown upside down in FIG. 3).

The container 46 must provide a passageway for thermocouples for the purpose of measuring the temperature in the location at which it is necessary to maintain the solidification front as well as the temperature gradients on each side of said front. In the form of construction which is illustrated, provision is made for three thermocouples (which are not shown in FIGS. 1 and 2), the arrangement of which is shown in FIG. 4. These thermocouples traverse the container through a lateral slit 64 formed in a lateral face of the casing and terminate near the wall of the cradle 58 after having traversed a screen 60 through a slit 66.

As shown in FIG. 4, the central thermocouple 68 is mounted so as to measure the absolute value of temperature at the point at which it is located: this thermocouple will be employed as element for detecting the regulation of the holding furnace l2 which is intended to produce action so as to maintain the temperature at the level of said furnace at a value equal to the solidification point of the material to be treated. On the contrary, the thermocouple 70 is mounted differentially so as to produce between the points 72 and 74 a signal representing the temperature difference between the thermocouples 68 and 70 and therefore the gradient in the liquid phase of the material in proximity to the solidification interface.

The third thermocouple 76 is mounted in a manner which is similar to the thermocouple 70 and delivers between the points 78 and 80 a signal which is proportional to the temperature gradient within the solid phase in proximity to the solidification interface.

It is preferable to mount the thermocouples on an adjustable support which is not illustrated in order that the distances between them can be modified and therefore in order to measure the gradients over intervals of greater of lesser value, for example between 1 mm. and mm.

The regulation device which is associated with the device illustrated in FIGS. l and 2 and assumed to be equipped with resistance furnaces l2 and [4 can be of the type which will now be described by way of example with reference to FIG. 4.

The regulation of temperature at the level of the furnace 12 comprises a conventional electronic device 82, preferably of the type known as a "proportional-integral-differential assembly, which regulates the dissipated power in the resistors of the holding furnace 12 so as to maintain the temperature detected by the thermocouple 68 at a constant value which is equal to the solidification point.

Regulation of the temperature gradient within the liquid phase is carried out by displacement of the main furnace 14. The corresponding device comprises an electronic circuit 84 (as shown in FIG. 4), the detecting element of which is constituted by the thermocouple 70 which is mounted differentially with the thermocouple 68. The thermal power which is dissipated within the resistors of the furnace is first set permanently at a constant value corresponding to the establishment of a suitable temperature gradient in respect of a mean position of the furnace l4 and the subsequent maintenance of said gradient is ensured by the displacements of the furnace 14. The regulating device 84 in which the gradient to be established is set up controls the servomotor 18 so as to move the furnace l4 closer to the furnace 12 if the gradient is too low. The regulating chain 84 can comprise, for example, an amplifier followed by a galvanometer having two adjustable index relays. The interval between the current densities corresponding to the actuation of the two relays is chosen at a value such that the rates of alternating motion of the main furnace 14 are not too high.

The temperature gradient within the solid phase in proximity to the interface is maintained at a substantially constant value by means of an electronic system 86 and the detecting element of this latter is constituted by the thermocouple 76 which is mounted differentially with the thermocouple 68. The electronic system 86 which can also be of conventional type controls a valve 88 for adjusting the rate of flow of coo mt through the sleeve which constitutes the furnace l6.

Experience has shown that the regulating device which has just been described has very fast action since it eliminates any problem of thermal inertia of the furnace, is wholly independent of any possible variations in temperature of the main furnace 14 which can be stabilized by coarse control, and is not influenced by variations in heat transfer within the container (such variations being due to the finite length of this latter) since they are compensated practically instantaneously by said container, these results being achieved notwithstanding the fact that the device remains of extremely simple design. Since the regulation only takes place as a function of the temperature gradient in the zone in which said gradient plays an essential part, the invention removes the need to heat the entire liquid phase to an excessive temperature and the power consumption is reduced accordingly. This advantage is further enhanced by the dissymmetrical temperature distribution within the furnace [4: in addition to the economy achieved in heat to be transmitted to the material by the furnace 14, there is obtained correlatively an economy in the heat to be removed by the coolant.

The rate of withdrawal of the container can be of any desired value provided that it remains lower than the maximum speed of the main furnace 14. Moreover, it will be understood that it must be possible for all the movements to take place without inducing undesirable vibrations within the container. The main furnace must have a length which is slightly greater than that ofthe mass of material in liquid phase.

The invention evidently permits of adaptation to a number of different alternative forms. In particular, it is clearly possible to dissociate the extraction function of the container from the cooling function and to employ a cold furnace which is either stationary or movable with respect to the holding furnace independently of the traction rod which is attached to the container: in principle, this arrangement offers an additional regulation parameter but is not usually necessary in practice.

What we claim is:

l. A device for displacing a solid-liquid interface along an elongated body of material in order to unidirectionally grow the solid phase, comprising an elongated container for said body of material; means for displacing said container along its longitudinal direction; a first heating oven located on the path of said container and having temperature regulating means for maintaining the temperature of the body portion in said oven at a value close to the point of change of phase; a second oven having higher power and thermal inertia than those of the first oven, said second oven being located on said path upstream of the first oven in the direction of movement for maintaining the body within the container in said second oven at a temperature above the point of change of phase; means for displacing said second oven relative to to the first along said longitudinal direction; and regulating means which adjust the position of said second oven for maintaining the temperature gradient at a constant value within the liquid phase in proximity to the solidification interface.

2. A device according to claim 1, wherein the second oven is of a type providing a temperature distribution having a maximum value which is offset towards the first oven with respect to the midplane of the second oven so as to provide a substantial temperature gradient within the liquid phase in proximity to the solidification interface without thereby entailing the need to heat the entire liquid phase to said maximum temperature.

3. A device according to claim 1 comprising a third oven or a heat sink having a temperature which is lower than that of the point of change of phase, said third oven being remote from the second oven with respect to the first.

4. A device according to claim 3, wherein said third oven is constituted by a sleeve which is attached to the container and through which is circulated a variable flow of coolant.

5. A device according to claim 1 comprising means for measuring the temperature of the material at the level of the first oven, said means being associated with a device for regulating the dissipated puwer within the first oven to a value such that ty to the interface, said means being associated with a device said temperature xhnuid remain constant for regulating the position ofthe second oven so as to maintain 6 A device according to claim comprising means for measaid gradient within a predetermined range surmg the thermal gradient within the liquid phase in proximi- 

2. A device according to claim 1, wherein the second oven is of a type providing a temperature distribution having a maximum value which is offset towards the first oven with respect to the midplane of the second oven so as to provide a substantial temperature gradient within the liquid phase in proximity to the solidification interface without thereby entailing the need to heat the entire liquid phase to said maximum temperature.
 3. A device according to claim 1 comprising a third oven or a heat sink having a temperature which is lower than that of the point of change of phase, said third oven being remote from the second oven with respect to the first.
 4. A device according to claim 3, wherein said third oven is constituted by a sleeve which is attached to the container and through which is circulated a variable flow of coolant.
 5. A device according to claim 1 comprising means for measuring the temperature of the material at the level of the first oven, said means being associated with a device for regulating the dissipated power within the first oven to a value such that said temperature should remain constant.
 6. A device according to claim 5 comprising means for measuring the thermal gradient within the liquid phase in proximity to the interface, said means being associated with a device for regulating the position of the second oven so as to maintain said gradient within a predetermined range. 